1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device and, more particularly, to a method of forming a resist pattern in a photolithography process. The invention further relates to a developing apparatus using the fabrication method. Incidentally, the term “semiconductor device” used herein generally represents a semiconductor device of the type which has a circuit construction made of thin film transistors (hereinafter abbreviated to TFTs), and display devices such as an active matrix type liquid crystal display device or an EL (abbreviation for Electroluminescence) display device are included in the category of the semiconductor device.
2. Description of the Related Art
In recent years, active matrix types of liquid crystal display devices having circuit constructions including TFTs are applied to the display screens of personal computers and television sets, and various products are in circulation in the market. In addition, active matrix type EL display devices of the self-luminous type which do not need backlights are considered to be advantageous for reductions in the thickness of display parts and reductions in production costs, and various manufacturers are conducting intensive research and development of associated products. In the fabrication of a display device such as an active matrix type of liquid crystal display device or EL display device, similarly to the case of an LSI (abbreviation for Large Scale Integrated Circuit) fabricating process, a thin film depositing step such as a CVD step, a photolithography step, an etching step and a resist removing step are repeatedly performed to form a fine device pattern. The photolithography step is the step of forming a resist pattern which becomes a base for the device pattern, the etching step is the device pattern forming step of performing etching processing on an underlying-layer film by using the resist pattern as a mask, and the resist removing step is the step of removing an unnecessary resist pattern after etching.
The above-described photolithography step is the step of forming a resist pattern which becomes a mask for etching, and in the process of fabricating the display device, a diazonaphthoquinone (hereinafter abbreviated to DNQ)-novolac resin type of positive resist is generally used as a resist material. An aligner for the photolithography step uses a 1:1 projection aligner (specifically, for example, MPA made by Canon) which uses multiple-wavelength light including G-line (436 nm), H-line (405 nm) and I-line (365 nm) which are spectral light of a super high pressure mercury lamp, or a 1:1 projection aligner using single-wavelength light abbreviated as a 1:1 stepper of G-line or I-line of a super high pressure mercury lamp. Specific processes of the photolithography step differ between the case where the 1:1 projection aligner of multiple-wavelength light is used and the case where the 1:1 projection aligner of single-wavelength light is used. The photolithography step in the case where the 1:1 projection aligner of multiple-wavelength light is used includes a series of steps: [resist coating]→[prebake (approximately at 100° C.)]→[exposure]→[development]→[postbake (approximately at 120° C.)]. The photolithography step in the case where the 1:1 projection aligner of single-wavelength light is used includes a series of steps: [resist coating]→[prebake (approximately at 100° C.)]→[exposure]→[bake after exposure (hereinafter abbreviated to PEB) (approximately at 120° C.)]→[development]→[postbake (approximately at 120° C.)], and is characterized in that the PEB processing is introduced after exposure.
Incidentally, in the case where the 1:1 projection aligner of single-wavelength light is used, the reason why the PEB processing is introduced after exposure is to prevent interference fringes unsuited to the formation of a fine pattern from being formed on the sidewall of a resist pattern. Namely, it is known that in the case where a resist pattern is exposed by the 1:1 projection aligner of single-wavelength light, owing to the single-wavelength of exposure light, the phenomenon that light intensity varies along the depth direction of a resist film occurs in the interior thereof in an exposed region as the result of the interference between light incident on a substrate and light reflected from the substrate. As a result, the variation phenomenon of light intensity incurs the phenomenon that the concentration of indene carboxylic acid (a photochemical reaction product from a DNQ photosensitizer) varies along the depth direction, and if the PEB processing is not performed after exposure, interference fringes are formed on the sidewall of a resist pattern. The PEB processing after exposure has the action of thermally diffusing the variations of the concentration of indene carboxylic acid that exist in the interior of the resist film in the exposed region, and uniformizing the concentration variations along the depth direction. Accordingly, the PEB processing can prevent the occurrence of interference fringes on the sidewall of the resist pattern after development. Some people skilled in the art have advanced the theory that the PEB processing after exposure is also effective on a standing-wave effect which is the phenomenon that the size of the resist pattern periodically varies with variations in the thickness of the resist film. For such reasons, in the case where the 1:1 projection aligner of single-wavelength light is used, the PEB processing is generally introduced after exposure. On the other hand, in the case where the 1:1 projection aligner of multiple-wavelength light is used, multiple-wavelength light (g-line, h-line and i-line of a super high pressure mercury lamp) is used as exposure light, and there hardly occurs the phenomenon that light intensity varies owing to the interference between light incident on a substrate and light reflected from the substrate. Accordingly, the PEB processing after exposure is basically unnecessary. However, since the introduction of the PEB processing does not cause a particular problem in terms of processes, the PEB processing may also be introduced.
In the formation of a fine pattern which is necessary for the fabrication of an LSI or the like, in terms of resolution, it is preferable that the shape of a resist pattern is generally close to a rectangle. On the other hand, in the case of fabrication of a display device, since the step of forming an etching pattern having a forwardly tapered shape is included, it is required to form a resist pattern having a small sidewall angle (sidewall angle: approximately 40-60 degrees) which disadvantageously affects resolution in the step of forming the etching pattern. The reason why the formation of a resist pattern having such a small sidewall angle is required is also influenced by the fact that the scaling of patterns in display devices presently has not advanced compared to LSIs and resolution is not very important. The formation of the resist pattern can be realized to some extent by a combination of an existing resist material of low resolution and an aligner, but according to the kind of taper etching step using a resist-receding method, it is necessary to form a taper portion of an etched pattern to elongate the size of the taper portion further more, so that it is necessary to form a resist pattern having a far smaller sidewall angle (for example, not greater than 50 degrees). As one example of the method of decreasing the sidewall angle of a resist pattern, there is a method of performing bake processing at a temperature not lower than glass transition point, but it is known that removing of resist patterns become more difficult along with raising of bake temperature.
The resist removing step for removing an unnecessary resist pattern will be described below. A resist pattern formed in the photolithography step serves as a mask for dry etching processing or wet etching processing, and after the completion of etching processing, it is necessary to remove an unnecessary resist pattern. For this reason, resist removing processing which includes an ashing step and a resist removing step is performed for the purpose of removing such an unnecessary resist pattern. The ashing step is the step of decomposing a resist pattern into carbon dioxide by means of oxygen plasma, and is a vapor-phase resist removing step. The resist removing step is the step of dipping a substrate after ashing processing into an organic resist removing solution adjusted to a predetermined temperature (approximately 60-90° C.) and dissolving and removing a resist pattern by using the dissolution action of the resist removing solution, and is a liquid-phase resist removing step.
In the resist removing step including the ashing step and the resist removing step, it is known that a resist pattern after dry etching processing becomes difficult to remove. When a resist pattern on a substrate passes through a dry etching step, the reaction of polymers which constitute a resist with an etching gas and the crosslinking reaction of the polymers proceed, and a deteriorated layer difficult to remove is produced on the surface of the resist pattern. The deteriorated layer has resistance to ashing and tends to prolong the time of ashing processing, and the speed of ashing is improved by adding a predetermined ratio of hydrogen or nitrogen to oxygen which is an ashing gas. Otherwise, the speed of ashing is also improved by adding a halogen gas such as CF4 to oxygen which is an ashing gas, but since there is the problem that a base substrate is etch-damaged in terms of the selectivity of the resist pattern to the base substrate, the ashing step is applied to a limited process.
In addition, in the resist removing step after the ashing step, resist removing capability is required to be improved, as by using a resist removing solution having a strong removing capability. However, it is known that a resist removing solution having a strong resist removing capability has a harmful effect of causing etch damage to the active layers of TFTs formed of a silicon-based semiconductor film, and a further improvement of the performance of the resist removing solution is desired. Incidentally, the problem of etch damage caused to the silicon-based semiconductor film by the resist removing solution is a phenomenon which occurs when the resist removing solution which exhibits strong alkaline as the result of the hygroscopic action of the resist removing solution comes into direct contact with the silicon-based semiconductor film, and various measures are being investigated from the viewpoints of both an improvement of the resist removing solution and a process improvement (such as a protective film is deposited on the surface of the silicon-based semiconductor film).